Deformable mirror method and apparatus including bimorph flexures and integrated drive

ABSTRACT

An apparatus comprising a substrate; and a platform elevated above the substrate and supported by curved flexures. The curvature of said flexures results substantially from variations in intrinsic residual stress within said flexures. In one embodiment the apparatus is a deformable mirror exhibiting low temperature-dependence, high stroke, high control resolution, large number of degrees of freedom, reduced pin count and small form-factor. Structures and methods of fabrication are disclosed that allow the elevation of mirror segments to remain substantially constant over a wide operating temperature range. Methods are also disclosed for integrating movable mirror segments with control and sense electronics to a produce small-form-factor deformable mirror.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/425,049 entitled Reduced Rotation MEMS DeformableMirror Apparatus and Method, and U.S. Provisional Patent Application No.60/425,051 entitled Deformable Mirror Method and Apparatus IncludingBimorph Flexures and Integrated Drive, both filed Nov. 8, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a methods and structures for elevating aplatform above a substrate and for producing a controlled motion of thatplatform. It also relates to MEMS deformable mirror (“DM”) arrays, andmore particularly to long-stroke MEMS deformable mirror arrays foradaptive optics applications.

2. Description of the Related Art

Adaptive optics (“AO”) refers to optical systems that adapt tocompensate for disadvantageous optical effects introduced by a mediumbetween an object and an image formed of that object. Horace W. Babcockproposed the concept of adaptive optics in 1953, in the context ofmirrors capable of being selectively deformed to correct an aberratedwavefront. See John W. Hardy, Adaptive optics for astronomicaltelescopes, Oxford series in optical and imaging sciences 16, OxfordUniversity Press, New York, 1998. Since then, deformable mirrors (DM)have been proposed for a variety of AO applications, although they haveyet to be implemented in many such proposed applications.

The general operation of a DM is shown schematically in FIG. 1, in whicha DM 100 reflects an aberrated wavefront 105, resulting in a desiredplanar wavefront 110. The DM shape is dynamically adapted to correct thepath-length variations of the inbound aberrated wavefront. That is, byselectively deforming the mirror to decrease or increase the path lengthfor specific portions of the aberrated wavefront, the aberrations in thereflected wavefront are corrected. The amount of local displacementneeded of the DM surface is generally approximately equal to half thepath-length variations in the aberrated wavefront. The exact scalefactor depends on the angle at which the aberrated wavefront strikes thedeformable mirror.

A prior art AO system is shown schematically in FIG. 2. This example isparticularly related to an astronomical telescope application, but thegeneral principles of AO shown here are illustrative of otherapplications. In FIG. 2, an aberrated wavefront 105 enters the opticalsystem 205 where it is modified as it reflects off a DM 100. Aberrationsin the wavefront reflected from the DM are the error signal for acomputer-controlled feedback loop. The reflected wavefront 110 enters adichroic beam splitter 220; the infrared wavelengths pass to a sciencecamera 225 and the visible wavelengths reflect toward a wavefront sensor230. The wavefront sensor measures the wavefront slope at discretepoints and sends these data to a wavefront reconstructor 235. Thewavefront reconstructor 235 determines the remaining wavefrontaberrations in the corrected wavefront. An actuator control block 240calculates actuator drive signals to correct the remaining wavefronterrors, which are sent from the block 240 to the DM 100, thus closingthe feedback loop. In this way, the DM is continuously driven in such away as to minimize the aberrations in the reflected wavefront, therebyimproving image resolution at the science camera.

AO systems have been proposed and demonstrated for improving resolutionin a number of imaging applications. In astronomy, for example, AO hasbeen used to correct aberrations introduced by motion of the atmosphere,allowing ground-based telescopes to exceed the resolution provided bythe Hubble Space Telescope under some observing conditions. In the fieldof vision science, AO has been shown to offer benefits, for example, forin-vivo retinal imaging in humans. Here, AO systems can compensate forthe aberrations introduced by the eye, improving lateral imageresolution by a factor of three and axial resolution by a factor of tenin confocal imagers. This has allowed individual cells to be resolved inliving retinal tissue, a capability that was not present before theadvent of AO.

In addition to improving image resolution, AO systems can be used toimprove confinement of a projected optical beam traveling through anaberrating medium. Examples of applications in this category arefree-space optical communication, optical data storage and retrieval,scanning retinal display, and laser-based retinal surgery.

A number of characteristics are commonly used to compare performance ofDM designs. Fill-factor is the fraction of the DM aperture that isactively used to correct wavefront aberrations. Mirror stroke is theamount of out-of-plane deformation that can be induced in the DMsurface. The number of degrees-of-freedom is a measure of the spatialcomplexity of the surface shapes the DM is capable of assuming and isrelated to the number of individual actuators that are used to deformthe mirror surface. DM aperture diameter, DM device size, controlresolution, operating temperature range, power consumption, frequencyresponse and price are also generally considered when selecting a DM fora given application. For example, astronomical imaging typicallyrequires mirror stroke in the range of a few micrometers, frequencyresponses in the kilohertz range and aperture sizes on the order of afew centimeters to a few meters. Systems for imaging structures in thehuman eye, by contrast, generally require mirror stroke on the order of10 micrometers or greater, frequency responses in the tens to hundredsof Hertz range, and aperture sizes on the order of one centimeter orless.

Despite the advantages outlined above, AO has not been universallyadopted, even in the aforementioned applications. Two important factorsthat have impeded the widespread adoption of AO are the high cost andlimited stroke of available DMs.

DM designs can be broadly divided into two classes;continuous-face-sheet designs and segmented designs.Continuous-face-sheet DMs have a reflective surface that is continuousover their whole aperture. The surface is deformed using actuators,typically mounted behind it, that push or pull on it to achieve adesired deformation. This type of DM has been implemented, for example,by mounting an array of piezoelectric actuators to the rear surface of asomewhat flexible glass or ceramic mirror. Because the optical surfaceis continuous and rather inelastic, large actuation forces are requiredto deform the mirror, and the resulting mirror stroke is small,typically less than 5 micrometers. The continuous surface also meansthat the deformation produced by each actuator is not tightly confinedto the area of the mirror directly connected to it, but instead mayextend across the whole mirror aperture, making precise control of theoverall mirror deformation problematic. Because of the way they areconstructed, such DMs are also comparatively large, having apertures onthe order of 50 mm or greater. This large size precludes theirdeployment in many optical systems that might otherwise benefit from AO.Their fabrication methods also make these DMs expensive to manufactureand do not permit easy integration of control electronics into the DMstructure.

A number of continuous-face-sheet DMs using microfabrication techniquesthat offer the potential to reduce DM size and cost have been created.Vdovin and Sarro, in “Flexible mirror micromahined in silicon”, AppliedOptics, vol. 34, no. 16 (1995), disclose a DM fabricated by assembling ametalcoated silicon nitride membrane above an array of electrodes thatare used to deform the membrane by electrostatic attraction.

Bifano et al. disclose an alternative microfabricatedcontinuous-face-sheet DM in “Microelectromechanical Deformable Mirrors”,IEEE Journal of Selected Topics in Quantum Electronics, vol. 5 no. 1(1999). Their design relies on the removal of a sacrificial layer tocreate cavities underneath the mirror surface that define the maximumtravel range of each mirror actuator. U.S. Pat. No. 6,384,952 to Clarket al. (2002) discloses a continuous-face-sheet DM that employs amirrored membrane fabricated, for example, from metalcoated siliconnitride and actuated by an array of vertical comb actuators disposedunderneath the membrane. Use of vertical comb actuators can providehigher force for a given applied voltage than the parallel plateelectrostatic actuators used in other continuous-face-sheet designs.

In contrast to the continuous-face-sheet designs discussed above,segmented DM designs divide the DM aperture into a number of generallyplanar mirror segments, the angle and height of each segment beingcontrolled by a number of actuators. Segmented designs are advantageousin that they allow the area of influence of each actuator to be tightlyconfined, simplifying the problem of driving the mirror to a particulardesired deformation. Segmenting the mirror surface also eliminates theneed to deform a comparatively inflexible optical reflector to produce adesired DM surface shape. Rather, the individual mirror segments aretilted, raised and lowered to form a piecewise approximation of whateverdeformation is required to correct the aberrations of the incomingwavefront. Segmenting the surface can therefore result in a lower forcerequirement for a given surface deformation, enabling the high-strokeDMs that are needed for many AO applications.

A number of inventors have disclosed segmented DM designs that may beconstructed using microfabrication techniques. U.S. Pat. No. 6,175,443to Aksyuk et al. (2001) discloses an array of conductive mirrorelements, connected together by linking members that act as supports,suspending the mirror array above an actuating electrode. These linkingmembers also serve to keep the mirror array in an approximately planarconfiguration when no actuating voltage is applied. Energizing theelectrode results in an attractive force between it and the mirrorsegments, deforming the array into a curved configuration.

U.S. Pat. No. 6,028,689 to Michalicek et al. (2000) discloses an arrayof mirror segments attached to a substrate by posts, each segmentcapable of tilting about two axes and also moving vertically,perpendicular to the array, under the influence of applied controlvoltages.

U.S. Pat. No. 6,545,385 to Miller et al. (2003) discloses methods forelevating a mirror segment above a substrate by supporting it onflexible members that can bend up out of the substrate plane. Thisprovides a large cavity underneath the mirror segment, not limited bythe thickness of the sacrificial materials used in its fabrication, andoffering the potential for large mirror stroke.

Helmbrecht, in “Micrmirror Arrays for Adaptive Optics”, PhD. Thesis,University of California, Berkeley (2002), discloses a segmented DM foruse in AO applications, that exhibits high fill-factor, high mirrorquality and offers the potential for high mirror stroke.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide improvedmethods and structures for elevating a platform above a substrate andfor precisely controlling the tip, tilt and piston motion of thatplatform.

A further object of the invention is to provide a high-degree-of-freedomDM which can be used to compensate for large optical wavefrontaberrations, without the need for temperature control or monitoring.

Another object of the invention is to provide a high-degree-of-freedom,high-stroke DM with integrated control electronics in a smallform-factor configuration.

A further object of the invention is to provide ahigh-degree-of-freedom, high-stroke DM with integrated sense electronicsin a small form-factor configuration.

Yet another object is to provide a high-degree-of-freedom DM with agreatly reduced control-pin count.

A further object of the invention is to provide a small-form-factor DMthat can be used in clinical ophthalmic instruments to correct wavefrontaberrations of the human eye.

A further object of the invention is to provide ahigh-degree-of-freedom, high-stroke DM that can be fabricated at lowcost.

A further object of the invention is to provide atemperature-insensitive, high-fill-factor, segmented piston-tip-tilt DM,having segments with improved optical flatness.

Yet another object of the invention is to provide a highly-reliable DM,capable of operating over many millions of actuation cycles.

A further object of the invention is to provide a high-degree-of-freedomDM comprising actuators that may be operated largely independently, inorder to provide correction for different areas of an optical wavefront.

A further object of the invention is to provide a DM that can be batchfabricated using IC-compatible fabrication methods and materials.

A further object of the invention is to provide a high-degree-of-freedomDM with reduced power consumption.

In accordance with the above objects, the invention, roughly describedcomprises an apparatus including a substrate and a platform elevatedabove the substrate and supported by curved flexures, wherein thecurvature of said flexures results substantially from variations inintrinsic residual stress within said flexures.

In another embodiment, the invention comprises a tiled array of mirrorsegments, each supported by a number of curved flexures attached, at oneend, to the underside of the segment and, at the other end, to asubstrate. A number of independently addressable actuators are used toapply forces to each mirror segment, causing it to move in a controlledmanner. The application points of the actuating forces and the locationsof the support flexures are placed so as to allow each segment to betilted about two distinct axes substantially parallel to the substrateand translated along an axis substantially perpendicular to thesubstrate. The invention may optionally include electronic circuitsembedded in the substrate for the purpose of addressing the individualactuators and/or sensing the state of a given mirror segment. Theinvention includes methods and structures for improved flexures forsupporting and elevating the segments above the substrate. Moreparticularly, the invention provides methods and apparatus forfabricating mirror segments supported by curved flexures, the curvatureof which is induced, principally or entirely, by variations in intrinsicresidual stress through the thickness of the flexure material ormaterials. The invention also includes methods for separatelyfabricating the MEMS portion of the inventive apparatus and theelectronics portion, and then integrating the two to form the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Illustration of prior art use of a deformable mirror to correctan aberrated wavefront.

FIG. 2: Illustration of prior art adaptive optic (AO) system.

FIG. 3A: Partial cutaway perspective view of a first embodiment of theinvention.

FIG. 3B: Perspective view of the improved flexure according to a firstembodiment of the invention.

FIG. 4A: Flow diagram of the process steps required to fabricate a firstembodiment of the invention.

FIG. 4B: Schematic cross-sections through structures fabricated atvarious process steps in a first embodiment of the invention.

FIG. 5: Schematic cross-section through the MEMS structures for a singlemirror segment in a first embodiment of the invention.

FIG. 6: Schematic cross-section through the portion of the CMOSsubstrate underlying a single mirror segment in a first embodiment ofthe invention.

FIG. 7A: Schematic cross-section through a single mirror segment andunderlying structures before MEMS release and passivation layer removalin a first embodiment of the invention.

FIG. 7B: Schematic cross-section through a single mirror segment andunderlying structures after MEMS release and passivation layer removal,in a first embodiment of the invention.

FIG. 8: Table of coefficients of thermal expansion for several candidatematerials for construction of curved flexures.

FIG. 9: Partially exploded perspective view of a third embodiment of theinvention.

DETAILED DESCRIPTION

Methods and structures for elevating one or more platforms above asubstrate and for controlling the tip, tilt and piston motion of thoseplatforms with high precision are hereinafter described. Severalembodiments are described in which a plurality of such platforms aretiled to form a large-stroke segmented piston-tip-tilt deformablemirror.

FIG. 3A shows a partial cutaway perspective view of a first embodimentof a DM incorporating the improved methods and structures. The DM isformed on a substrate 300, which may be a silicon wafer or chipcontaining embedded addressing and sensing circuits (not shown). On topof the substrate 300 are formed a number of control electrodes 370 thatare electrically isolated from one another and electrically connected tothe embedded addressing and sensing circuits. In the first embodiment,the control electrodes 370 are arranged in groups of three and arerhombic in shape, so that the footprint of each group is essentiallyhexagonal. Disposed around each group of three control electrodes 370,are three conductive ground pads 310, fabricated from the same materialas the control electrodes 370. The ground pads 310 are electricallyisolated from the control electrodes 370 and electrically connected to aground plane or to circuits embedded in the substrate 300. Attached toone end of each ground pad 310 is a first anchor portion 350 of aflexure 320. The flexure, in the first embodiment comprises two layers,a first flexure layer 330 formed from conductive polycrystalline siliconand a second flexure layer 340 formed from silicon nitride (SixNy). Thefirst anchor portion 350 is both mechanically and electrically connectedto the ground pad 310 so that the conductive first flexure layer 330 isheld at the same electrical potential as the ground pad 310. The secondflexure layer 340 is rigidly attached to the underside of the firstflexure layer 330 and extends over a portion of the length of theflexure 320. The purpose of the second flexure layer is to provide aresidual stress difference between the top and bottom portions of theflexure 320, causing the flexure 320 to bend up out of the plane of thesubstrate 300.

The end of the flexure 320 opposite the first anchor portion 350terminates in a second anchor portion 360. FIG. 3B is a detailperspective view of one flexure 320, showing the first anchor portion350, the second anchor portion 360, the first flexure layer 330, thesecond flexure layer 340, and the ground pad 310 underlying the flexure.

Referring again to FIG. 3A, the second anchor portion 360 ismechanically and electrically connected to the underside of a mirrorsegment 380. The mirror segment is any one individual mirror of the DMdevice. Thus the mirror segment 380 is held at some elevation above thesubstrate 300. In the first embodiment, this elevation is on the orderof 50 micrometers. The mirror segment is electrically conductive andtherefore is held at the same potential as the ground pad 310. In thefirst embodiment, the mirror segment 380 is hexagonal in shape and isformed from a 20 micrometer-thick layer of single crystal silicon and iscoated on its top surface with an optical coating, which may be a highlyreflective metal layer. The mirror segment diameter in the firstembodiment is on the order of 500 micrometers.

For the sake of clarity, FIG. 3A shows only three mirror segments 380.However, an exemplary embodiment of the DM comprises an array of 121nominally identical elevated mirror segments 380 disposed over thesubstrate so as to form a larger, segmented mirror surface,approximately circular in outline and having inter-segment gaps of 5micrometers.

The following is a general overview of the process of the currentinvention for fabricating the first embodiment of the DM. The processinvolves separately fabricating the MEMS structure and the addressingand sensing circuits on two separate wafers, then assembling themtogether as shown in FIGS. 4A and 4B. FIG. 4A is a process flow diagramand FIG. 4B illustrates the corresponding structure at each step. Asshown at step 400, each mirror segment 380 is fabricated by reactive ionetching (RIE) the top single crystal silicon “device” region of a bondedsilicon-on-insulator wafer (BSOI). At step 405, the wafer is then coatedwith a sacrificial layer to fill the trenches left by the previous etch,provide a temporary support for various mechanical structures of the DM,and optionally to act as a dopant source for undoped polysiliconregions. This sacrificial layer might typically be phosphorus-dopedsilicate glass deposited by low pressure chemical vapor deposition(LPCVD). Alternatively, in cases where the sacrificial layer is notrequired to act as a dopant source, silicon oxide deposited by atetraethoxysilane (TEOS) process might be used.

As shown at step 410, the PSG region is next patterned to define theattachment points for the second anchor portions 360 of the flexures; insome instances the patterning may include an etching step. At step 415,a one micron undoped amorphous polysilicon layer and a PSG layer aredeposited by LPCVD and annealed at 950° C. for six hours to dope andtune the residual stress of the polysilicon layer to approximately −40MPa, where the negative sign denotes compressive stress. The top PSGlayer is then removed at step 420 using a wet hydrofluoric (HF) acidetch and the polysilicon layer is patterned and etched to define thefirst flexure layer 330 at step 425. Silicon nitride (SixNy) is thendeposited by LPCVD at step 430, and patterned and etched to define thesecond flexure layer 340 at step 435. At step 437, conductive metal padsare deposited, for example by electroplating, on to the first anchorportion 350 of the flexures. These metal pads will serve as theelectrical and mechanical attachment points between the flexures and thesubstrate 300.

FIG. 5 schematically illustrates a cross-section through the MEMSstructure 500 supporting a single mirror segment, completed up to thispoint and including the mirror segment 380, flexure 320 and thesacrificial layer 515, typically phosphorus-doped silicate glass (PSG).As compared with the structure shown at the last step 437 of FIG. 4B-1,the structure shown in FIG. 5 has been inverted in preparation forbonding to the electronics chip. In the first embodiment, the flexure320 is a two-layer structure with a first flexure layer 330 ofphosphorus-doped polysilicon, and a second flexure layer 340 of SixNy.Although not required in all embodiments, the MEMS device in the firstembodiment includes a temporary handle wafer (not shown in FIG. 5),typically 300 to 500 micrometers thick, used to support the MEMSstructure prior to release in a manner known in the art.

Continuing again with reference to process steps 445 onwards, shown inFIGS. 4A and 4B, drive circuitry in the form of an integrated circuit isnow introduced. This integrated circuit is the substrate 300 on whichthe flexures and mirror segments will be mounted. The substrate 300 istypically fabricated through separate processing in a conventionalmanner, for example using silicon CMOS techniques not shown here, andwell known in the art. As shown at step 445 in FIG. 4, the substrate 300is typically coated with a passivation layer to protect it from the MEMSrelease agent, which may for example be hydrofluoric acid. As shown instep 447 of FIG. 4, the passivation layer is patterned and etched toexpose bond sites on the substrate 300 that are electrically connectedto a ground plane or to underlying circuits. An electrically conductivebonding agent 610 is then deposited on these bond sites. FIG. 6 is aschematic cross-section through the substrate at the end of step 447,showing the locations of the control electrodes 370, the bonding agent610 and the wiring layer 625.

Continuing to refer to FIG. 4, at step 450 the MEMS structure 500,constructed as described above, is disposed over the substrate 300 andthe two are then bonded together. At this point the MEMS structure 500still includes the sacrificial layer 515.

At step 455 the handle wafer of the BSOI wafer is etched away from theMEMS mirror segment, after which the sacrificial layer is released fromthe MEMS structure as shown at step 460. The IC passivation layer isremoved at step 465, typically using an O2 plasma or appropriatesolvent. Finally, an optical coating is deposited on the top surface ofthe mirror segments, for example using a shadow-masked metalevaporation, in step 470. The resulting device is a completed,integrated DM. FIG. 7A shows a cross-section through a single mirrorsegment and underlying structures, after removal of the handle wafer atstep 455. FIG. 7B shows a cross section through the same structure atthe end of the fabrication process, after the MEMS sacrificial layer 515and circuitry passivation layer have been removed. The device includesthe following elements: IC portion or substrate 300 and MEMS structure500; on the IC portion are shown a control electrode 370, the bondingagent 610 and a wiring layer 625. On the MEMS portion 500 are shown themirror segment 380 and flexure 320, comprising the first flexure layer330 and second flexure layer 340.

One important aspect of the present invention is the above-describedpassivation layer. In the first embodiment of the invention, anelectrically-conductive contact must be established through thepassivation layer at the points where the MEMS structure 500 is bondedto the substrate 300. The bonding process can be any suitable processthat results in a conductive bond, for example gold to gold bonding. Toallow the bond material to be deposited onto the IC substrate 300, thepassivation layer is preferably patternable. In an exemplaryarrangement, the passivation layer is completely removable after theMEMS structure is released in a manner that will not damage the MEMSstructure. This passivation material may be a protective polymermaterial such as a polyimide or parylene.

Alternatively, the passivation material can be conductive so that uponremoval from the exposed surfaces, electrical contact between the ICsand MEMS element is maintained. The passivation material need not bepatterned before bonding as it is selectively removed, where not bondedto the MEMS structures, in the passivation layer removal process. Aconductive polymer or epoxy can be used, for example, EPO-TEK OH108-1 orother similar conductive epoxy made by Epoxy Technology, Inc., ofBillerca, Mass.

The present invention differs significantly from the prior art in thatit relies on the influence of IRS (as opposed to CTE) in the flexures toelevate the mirror segments above the substrate plane, to a much greaterdegree than has been found in the prior art. The “Coefficient of thermalexpansion” (“CTE”) describes the linear change in size of a material asa function of temperature, while “Intrinsic residual stress” (IRS)describes the stress in a material, which is dependent on the grainmorphology and crystalline defects of a material. This means that theelevation of the segments above the substrate can be far less sensitiveto changes in temperature than for comparable prior art devices. Thedeflection at the elevated end of each flexure is essentiallyproportional to the curvature of the flexure, which may be written asthe sum of two components; a first component proportional to theintrinsic residual stress in the flexure and a second componentproportional to the CTE mismatches in the flexure. In the firstembodiment of the invention, the first flexure layer is composed ofpolysilicon and the second flexure layer is composed of silicon nitride.This provides a flexure for which the IRS component is larger than theCTE component by a factor of approximately one thousand at normaloperating temperatures, for example in the range 0–100 degrees Celcius.

Many alternative embodiments of the flexure are possible in which thesecond flexure material is one with a CTE similar to that of the firstflexure material. If that first material is polysilicon, the secondmaterial can be a ceramic, such as SiC, or silicon nitride (SixNy), oreven polysilicon itself, deposited under different conditions so as toinduce a different grain structure and crystal defect concentration, andthus different IRS. FIG. 8 is a table that lists the CTE of some examplematerials.

In contrast to the prior art usage of nickel, SixNy is advantageousbecause it does not contaminate etchers as Ni does. SixNy is also easierto process because it is a standard IC material deposited by LPCVD. Theresidual stress of SixNy can be controlled by varying the ratios of thereactant gasses, deposition pressure, and the deposition temperature.For example, a layer deposited with a gas flow ratio of 1:3dichlorosilane to ammonia at 125 mTorr and 800 will yield astoichiometric film (Si3N4) with approximately 1 GPa of residual tensilestress, while 4:1 gas ratio at 140 mTorr and 835° C. will yield a filmcomposition near Si3N3 with approximately 280 MPa of residual tensilestress. To achieve the desired radius of curvature of the flexure,different SixNy stoichiometries can be used, the appropriate choice forwhich may be application-specific.

The first embodiment of the DM comprises a tiled array of mirrorsegments, supported on flexures and elevated approximately 50micrometers above the substrate. As described, the substrate containselectronic circuits used for controlling and sensing the tip, tilt andpiston motion of the segments. The circuits are controlled viaelectrical signals transmitted, for example, through bond pads on thesubstrate and generated, for example, by a microprocessor in a mannerwell known in the art. The control signals typically containinformation, generated by a wavefront reconstructor, about thecombination of tip, tilt and piston motions for each mirror segmentneeded to compensate for the wavefront aberrations at a given time.“Piston movement” is one of three types of movement used to describeactuation of a mirror segment, and describes translation normal to theplane of the DM aperture. “Tilt”, the second type of movement, ismovement about any first axis that is parallel to the plane of the DMaperture. “Tip”, the third type of movement, is movement about anysecond axis (not parallel to the first axis) that is also parallel tothe substrate.

The circuits embedded in the substrate 300 decode this information andtranslate it into a corresponding set of voltages that are applied tothe control electrodes disposed under each mirror segment. Theelectrical potential difference and resulting electrostatic forcebetween each mirror segment and its three control electrodes causes itto move in tip, tilt and piston, and assume a position and orientationdetermined by the voltages applied to the three electrodes. This abilityto independently orient and position each segment allows spatiallycomplex wavefront aberrations to be corrected by the DM. In someimplementations of the first embodiment, the substrate also containssense electronics that detect the tip, tilt and piston of each segment,for example by measuring the capacitance between the segment and itsthree control electrodes. Incorporation of sense electronics can improvethe resolution with which the segments can be controlled. Because theattractive force between a segment and its control electrodes increasesrapidly as the gap between them diminishes, the control voltages must belimited to avoid pulling segments into contact with the electrodes.Typically, the maximum operation voltage is chosen to be the voltagethat causes a segment to travel 25% of the elevation produced by theflexures. Therefore, the flexure elevation of 50 micrometers describedin the first embodiment results in a useable mirror stroke ofapproximately 12 micrometers.

In a second embodiment of the invention, the structure of the DM isidentical to the structure of the first embodiment, except that theground pads and control electrodes are formed on the MEMS part ratherthan the CMOS part. The appearance of the completed device isessentially identical to that of the first embodiment, illustrated inFIG. 3A.

Fabrication of the second embodiment proceeds in a manner identical tothat used for the first embodiment up to step 435 of FIG. 4B. Asacrificial layer is then deposited, patterned and etched to open upanchor points where the ground pads 310 will attach to the first anchorregions 350 of the flexures. A layer of polysilicon is then deposited,patterned and etched to define the ground pads 310 and the controlelectrodes 370. A layer of metal is then deposited, patterned and etchedso that it coats the surfaces of the ground pads 310 and controlelectrodes 370, but does not bridge unconnected structures.

The CMOS portion 300 of the device is fabricated in the same way as forthe first embodiment, but has bond sites in locations that correspond toboth the ground pads 310 and the control electrodes 370 of the MEMSstructure. The ground pad bond sites are electrically connected to aground plane or to circuits in the substrate 300, while the controlelectrode bond sites are connected to the appropriate control and sensecircuits within the substrate 300. The MEMS portion and the CMOS portionare bonded together using a film of anisotropic conductive polymer thatconducts only in a direction normal to the plane of the film. In thisembodiment, the anisotropic conductive polymer acts as both a bondingagent and a CMOS passivation layer. After bonding, the MEMS structuresare mechanically released, for example by HF etching, as in the firstembodiment. Because of the anisotropic nature of the polymer, it doesnot need to be removed from the DM and so the passivation layer removalstep is omitted for this embodiment. As for the first embodiment, thefinal step is the deposition of an optical coating on the top surface ofthe mirror segments.

The method of operation for the second embodiment is identical to thatfor the first embodiment.

FIG. 9 shows the mechanical structure of a DM according to the thirdembodiment of the invention, in a partially exploded perspective view.For the sake of clarity, FIG. 9 shows only a single piston-tip-tiltmirror segment. However, it will be clear to one skilled in the art thatmultiple such mirrors may be fabricated side-by-side on a singlesubstrate to form a segmented DM, as was described for the firstembodiment.

The third embodiment of the DM comprises a substrate 900, which may be asilicon wafer. On top of the substrate 900 are formed a number ofcontrol electrodes 960 that are electrically isolated from one anotherand electrically connected to conductive traces (not shown in FIG. 9)that may either be embedded in the substrate 900 or attached to thesurface of the substrate 900. These traces electrically connect thecontrol electrodes 960 directly to bond pads (not shown in FIG. 9) thatmay be disposed around the perimeter of the DM chip. The controlelectrodes 960 are arranged in groups of three and are rhombic in shape,so that the footprint of each group is essentially hexagonal.

Disposed around each group of three control electrodes 960, are threeconductive ground pads 910, fabricated from the same material as thecontrol electrodes 960. The ground pads 910 are electrically isolatedfrom the control electrodes 960 and electrically connected to a groundplane embedded in the substrate 900. Attached to one end of each groundpad 910 is a first anchor portion 950 of a flexure 920. The flexure, inthe third embodiment comprises two layers, a first flexure layer 930formed from conductive polycrystalline silicon and a second flexurelayer 940 formed from silicon nitride. The first anchor portion 950 isboth mechanically and electrically connected to the ground pad 910 sothat the conductive first flexure layer 930 is held at the samepotential as the ground pad 910. The second flexure layer 940 is rigidlyattached to the top side of the first flexure layer 930 and extends overa portion of the length of the flexure 920. The purpose of the secondflexure layer is to provide a residual stress difference between the topand bottom portions of the flexure 920, causing the flexure 920 to bendup out of the plane of the substrate 300.

The end of the flexure 920 opposite the first anchor portion 950 iselectrically and mechanically connected to a hexagonal platform 980. Aplatform bond site 990, fabricated from a metal, is electrically andmechanically connected to the platform. This platform bond site matchesup with a corresponding segment bond site, also fabricated from a metal,on the underside of a mirror segment 970. The segment bond site is notvisible in FIG. 9, since it is on the underside of the mirror segment970. In the fully assembled DM, the mirror segment 970 is mechanicallyand electrically connected to the platform 980 via these bond sites.Thus the mirror segment 970 is held at some elevation above thesubstrate 900. In the first embodiment, this elevation is on the orderof 50 micrometers. The mirror segment is electrically conductive andtherefore is held at the same potential as the ground pad 910. In thethird embodiment, the mirror segment 970 is hexagonal in shape and isformed from a 20 micrometer-thick layer of single crystal silicon and iscoated on its top surface with an optical coating, which may be a highlyreflective metal layer. The mirror segment diameter in the thirdembodiment is on the order of 500 micrometers.

In the third embodiment, the DM does not incorporate drive and senseelectronics, but does incorporate the improved bimorph flexure. Theactuator substrate 900 is fabricated in a method similar to that used tofabricate the MEMS portion of the first embodiment, but where thestarting material is a standard silicon wafer rather than a bonded SOIwafer. The ground pads 910, control electrodes 960, electrical tracesand bond pads are defined in a first undoped polysilicon layer,deposited on an insulating silicon nitride layer. Alternatively, thetraces could be fabricated in a buried layer beneath the electrodes thatis electrically isolated in all regions except areas that contact theelectrical traces to electrodes and bond pads. A phosphorous-dopedsilicate glass (PSG) sacrificial layer is then deposited, patterned andetched to open up regions where the first anchor portion 950 of theflexures will connect to the ground pads 910. A second undoped amorphouspolysilicon layer is then deposited followed by a PSG layer. The waferis annealed at 950° C. for six hours to dope and tune the residualstress of the second polysilicon layer to approximately −40 MPa. In thisstep, the sacrificial PSG layer also dopes the first layer ofpolysilicon. The top PSG layer is then removed using a wet HF acid etchand the second polysilicon layer is patterned and etched to define thefirst flexure layers 930 and platforms 980. A layer of silicon nitrideis then deposited, patterned and etched to define the second flexurelayers 940, after which a low-temperature oxide (LTO) is deposited byLPCVD to protect the structures from a later etch. The LTO layer isetched and a metal layer is selectively deposited, for example byelectroplating, to form the bond sites 990 and bond pads disposed aroundthe perimeter of the DM chip.

The mirror segments 970 are formed on a separate wafer, typically a BSOIwafer with a 20 micrometer thick device layer. The mirror segments aredefined using deep reactive ion etching, followed by deposition of asacrificial layer (typically PSG) that refills the trenches between thesegments. The sacrificial layer is then patterned and etched to clearaccess holes for bond sites that match those deposited on the actuatorsubstrate 900. A metal layer is then selectively deposited, for exampleby evaporation and lift-off, to form the segment bond sites that will bejoined to the corresponding platform bond sites 990.

The actuators and mirror segments are then assembled and bondedtogether, for example using gold to gold bonding. The mirror-segmenthandle wafer is then removed in a manner known to those skilled in theart, and the sacrificial layers are removed, for example by HF etching,to allow the flexures to lift the mirror segments 970 above thesubstrate 900. Finally, an optical coating is deposited on the topsurface of the mirror segments.

The third embodiment is operated in a manner similar to the firstembodiment, with the exception that the control voltages used to set theorientation and piston of the mirror segments are generated by driverelectronics on a chip or board that is physically separate from the DMchip. The control electrodes for each mirror segment are connected tothe outputs of the drive electronics for example via bond wireselectrically connected to the bond pads disposed around the edges of theDM chip.

Accordingly, the invention provides improved methods and structures forelevating a number of platforms above a substrate and for controllingthe piston, tip and tilt motions of those platforms. The resultingstructures feature low temperature dependence, small size and powerconsumption and high control precision. The methods and structures maybe used to construct an improved deformable mirror (DM) that featureslow temperature dependence, high fill-factor, high control resolutionand large stroke, and which can be fabricated in a small form-factor atlow cost. The ability to integrate drive and sense electronics on thesame chip as the mirror segments allows DMs with large numbers ofactuators to be realized. The structures and methods for producingtemperature-insensitive elevated mirror segments and the structures andmethods for assembling the mirror segments on to control and senseelectronics can be applied separately or in combination.

Although the description above contains many specificities, these shouldnot be construed as limiting the scope of the invention but as merelyproviding illustrations of some of the possible embodiments of thisinvention. For example, the mirror segments can have other shapes, suchas square, rectangular, triangular etc.; the mirror segments can besupported by different numbers of flexures; the flexures can beconstructed from any number of materials and comprise any number oflayers, provided their curvature is predominantly caused by IRS, ratherthan CTE differences; the tip, tilt and piston of the mirror segmentscan be controlled by varying the duty cycle of an AC signal applied tothe control electrodes rather than the magnitude of an applied DCsignal; the thicknesses of the layers that comprise the DM can bevaried; the diameters or widths of features such as the mirror segments,flexures and control electrodes can be varied; the number and placementof the control electrodes under each segment can be changed; theelevation of the mirror segments above the substrate can be altered; theactuators need not be electrostatic but could be, for example,piezoelectric or magnetic; the gaps between mirror segments can bechanged; different reflective coatings including both metallic anddielectric coatings can be deposited on the top surface of the segments;different materials and methods can be used to bond the MEMS portion tothe CMOS portion; different passivation materials can be used to protectthe CMOS circuits during MEMS release; the number of mirror segmentscomprising the DM can be varied, etc.

While numerous specific details have been set forth in order to providea thorough understanding of the present invention, numerous aspects ofthe present invention may be practiced with only some of these details.In addition, certain process operations and related details which areknown in the art have not been described in detail in order not tounnecessarily obscure the present invention.

Thus the scope of the invention should be determined by the appendedclaims and their legal equivalents, rather than by the examples given.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A microelectromechanical (MEMS) structure on a substrate, comprising:a platform connected with a set of one or more bimorph flexures; and theset of bimorph flexures connecting the platform with the substrate, eachbimorph flexure comprising a first layer comprised of a first materialand a second layer comprised of a second material, the first and secondmaterials having particular intrinsic residual stress (IRS)characteristics and coefficients of thermal expansion (CTEs), eachbimorph flexure having a curvature resulting from a first componentproportional to the difference in IRS characteristics of the first andsecond materials and a second component proportional to the differencein CTEs of the first and second materials, the first component beinglarger than the second component.
 2. The MEMS structure of claim 1,wherein the curvature of each bimorph flexure results predominantly fromthe first component.
 3. The MEMS structure of claim 1, wherein thecurvature of each bimorph flexure is not substantially sensitive tochanges in temperature.
 4. The MEMS structure of claim 1, wherein, thefirst component is larger than the second component by a factor ofapproximately one thousand or more.
 5. The MEMS structure of claim 4,wherein the first component is larger than the second component by afactor of approximately one thousand or more at normal operatingtemperatures of the MEMS structure.
 6. The MEMS structure of claim 1,wherein the first material comprises silicon and the second materialcomprises silicon nitride, or the first material comprises polysiliconand the second material comprises ceramic, SiC, or silicon nitride(SixNy).
 7. The MEMS structure of claim 1, wherein the second layerextends over a portion of the first layer that is less than the entirelength of the first layer, the second layer being affixed to the firstlayer along the entire length of the second layer.
 8. The MEMS structureof claim 7, wherein the second layer provides a residual stressdifference between the top and bottom portions of the first layer. 9.The MEMS structure of claim 1, wherein: the set of bimorph flexureselevates the platform above the substrate; and the platform is anactuator segment or mirror segment.
 10. The MEMS structure of claim 1,wherein the first and second layers are formed under conditions thatproduce substantially different IRS characteristics in the first andsecond materials.
 11. The MEMS structure of claim 1, wherein the firstand second materials comprise polysilicon.